Optical waveguides are structures that confine the optical field in one or two spatial dimensions and allow propagation of light in the third dimension. Confinement of the light is achieved by surrounding a core region of given refractive index with materials of lower refractive index to allow light in the core to be totally internally reflected at the interface with the surrounding materials. The geometric size and shape of the core and surrounding regions, as well as their respective refractive indices, define the distribution of light energy and the light propagation velocity in the waveguide.
Integrated waveguide structures are attractive because they are capable of integrating complex functionalities into a relatively small chip. The basic building block of integrated optics is essentially the waveguide circuit provided in a single plane parallel to a supporting wafer or substrate. A variety of planar fabrication techniques have been adopted from the semiconductor industry for fabricating integrated optic devices because such techniques are well established and understood. The typical process of fabricating integrated optic devices starts with layer deposition by, for example, molecular beam epitaxy (MBE), metalographic vapor phase epitaxy (MOPE), flame hydrolysis deposition (FHD), chemical vapor deposition (CVD), or spin coating of polymers or sol-gels. The layer is then patterned by, for example, reactive ion etching (RIE), reactive ion beam etching (RIBE), or ultra-violet (UV) patterning followed by development of the exposed layer by a wet dissolution process. The patterned layer, which forms the core, is then typically covered with a cladding layer by using one of the layer deposition processes mentioned above.
An optical waveguide realized by planar techniques typically includes a generally rectangular core region of a given refractive index surrounded by a cladding material of a lower refractive index. The thickness (y-dimension) of the core is defined by the layer deposition process employed, or in some instances a diffusion step, and is thus typically constant over the chip area within the limitations of the process equipment. Only the lateral dimension (x-direction) of the core region can easily be varied by using an appropriately designed photomask for the planar lithography involved in the layer patterning step, which also defines the layout and routing of the waveguides formed on the optical chip.
The number of waveguides, or channels, provided on an integrated optical chip is limited by the chip area, the amount of horizontal spacing provided between the channels, and the minimum bending radii that produces an acceptable amount of loss within the bend. The spacing between channels can be limited, for example, by the geometry of a fiber ribbon to be interfaced with an input and/or output facet of the optical chip or by the spacing required to prevent unwanted coupling between individual channels.
Waveguide structures that exhibit strong light confinement to the core, the so called “strongly guiding” waveguide structures, offer the possibility of smaller bending radii, as well as more closely spaced waveguides, than waveguide structures with weak light confinement. As a result, strongly guiding waveguide structures offer the possibility of realizing smaller devices and increased packing density. A problem arises, however, when strongly guiding waveguide structures have to be interfaced with standard fibers employed in optical communication networks. Strongly guiding waveguide structures that only support propagation of the fundamental mode (a strong requirement in most optical communication networks) have a much smaller mode-field diameter and larger Numerical Aperture than standard fibers used in optical communication networks. This mismatch reduces coupling efficiency and causes strong losses at the optical interface when light is launched from the fiber into the waveguide or vice versa.
Currently known waveguide technologies do not adequately address the competing goals of achieving increased packing density and low loss interfaces with standard optical communication network fibers. One attempt has been to provide a lateral taper at the end of the waveguide, as illustrated in FIG. 1, therefore increasing the lateral dimension of the waveguide mode. This design can be realized by using a modified mask layout with standard planar processing techniques, therefore ensuring good production yield. The disadvantage of this design is threefold: it does not address the mismatch in vertical direction, the structure will not be single-mode in the horizontal direction in the tapered region, and the Fresnel loss at the interface from the high-index waveguide to the low-index fiber remains unchanged.
More advanced structures using a three dimensional adiabatic taper, such as illustrated in FIG. 2, have been demonstrated, which deviate from standard planar processing by attempting to either deposit layers with thickness variations and index variations, or to locally modify parts of the structure. These structures typically transform the guide from a large core, low Δn type at the fiber input to a small core, high Δn type in the optical circuit. However, currently known approaches have the disadvantage of requiring non-standard fabrication techniques, which are hard to control and thus adversely affect the overall production yield. For example, the structure shown in FIG. 2 is accomplished through a modification of the standard flame hydrolysis deposition technique. The gas composition and scan pattern are gradually varied to deposit a thinner, higher Δn guiding layer near the center of the circuit. The core width is then varied by lithography. Further, the patterned core may then be over-coated with a cladding layer as illustrated. In another approach to forming an adiabatic taper, a small-core, high Δn waveguide is first fabricated with uniform parameters, and then modified by local diffusion of dopants included in the core so as to radially expand the core. This is done by clamping one end of the substrate in a cooled holder and heating the other to a high temperature. In yet another approach, a small-core, high Δn type waveguide is fabricated that is physically tapered to create a larger mode field at the interface. As illustrated in FIG. 3, this taper has to be applied in both the x and y-directions of the core to maintain a circular field shape. While the lateral size variation can be achieved relatively easily using standard planar techniques, the taper in the core thickness is very hard to control in the manufacturing process.
While the first approach has the advantage of not requiring a modification of the vertical core size, thus offering higher production yields, the small optical improvement, as discussed above, limits its usefulness. In view of the difficulties of physically tapering the core in the y-direction consistently, the second and fourth approaches described above are not widely practiced. The third approach has also not taken hold due to the high temperatures required to locally diffuse the dopants in the core, in addition to the issues created by applying a large thermal gradient across the optical chip by heating one end of the optical chip while cooling the other end.
In view of the shortcomings of current interface technologies, the desire to achieve a high coupling efficiency between integrated waveguides and standard fibers usually defines the waveguide dimension and guiding strength. As a result, most integrated waveguides manufactured today are weakly guiding and have a relatively large mode-field diameter to match the mode-field diameter of standard optical communication network fibers.
Another approach for increasing packing density is to stack the waveguide structures to make three-dimensional integrated optic devices. Basically, this approach relies on the use of two or more waveguiding layers with vertical separation between each. However, if adjacent waveguiding layers are coupled together by a directional coupler, the number of waveguides that may be provided in a particular waveguiding layer is severely limited by the number of waveguides included in the immediately adjacent waveguiding layer. This is because the vertical separation between the guiding layers must be defined to permit evanescent coupling between select guides in adjacent layers. As a result, however, waveguides in the adjacent guiding layers must be carefully laid out to avoid crossing paths or from even coming too close to the same path in order prevent crosstalk between channels in the respective guiding layers.